Long-lived rotary ball bearing for reciprocating applications and method of lubricating same

ABSTRACT

A rotary bearing includes concentric radially spaced-apart inner, intermediate and outer rings whose opposing surfaces define inner and outer ball tracks. An inner array of balls roll along the inner track and an outer array of balls roll along the outer track and a viscous lubricant is present between the rings. When the inner and outer rings are angularly reciprocated relative to one another and that reciprocation is changed, the intermediate ring is moved intermittently in a selected direction about the rotation axis so that the balls recirculate about the axis and redistribute lubricant along the tracks.

RELATED APPLICATION

This application is a continuation of Ser. No. 09/342,755, filed Jun.29, 1999, now abandoned which is a continuation-in-part of Ser. No.09/274,402, filed Mar. 23, 1999, now U.S. Pat. No. 6,196,725.

BACKGROUND OF THE INVENTION

This invention relates to a rotary ball bearing. It relates especiallyto a bearing used to facilitate reciprocating rotary motion of a shaftor the like and to a method of distributing or replenishing lubricant insuch a bearing.

Typically, a rotary ball bearing for a reciprocating rotary shaft or thelike is constructed with two rings, namely an inner ring mounted to theshaft and an outer ring mounted to a stationary support or housing. Thetwo rings define opposing races and are separated by a circular array ofballs. Relative rotation of the rings results in the rolling of theballs along the races in the rings. In order to reduce rolling frictionand to minimize wear of the bearing parts, the bearing is normallylubricated with a viscous lubricant such as oil or grease which occupiesthe spaces between the balls and the walls of the races. If the bearingis pre-loaded axially so that the internal clearances between the partsof the bearing are more or less removed, the balls are constrainedagainst “skidding” in their races during normal operation of thebearing. In other words, the initial relationship between the balls andthe races is fixed. Thus, if one bearing ring is rotated relative to theother back and forth through a small angle in reciprocation, each ballof the bearing rolls over a definite portion of the race in each ringand is constrained to roll over these same small areas of the races aslong as the bearing remains in use.

Reciprocating rotary shafts with bearings such as this are often used tomove a mirror used in beam scanning or steering applications. Theseapplications include electronic manufacturing and repair operations inwhich a laser beam is directed to perform tasks such as the profiling,marking, cutting, drilling, and trimming of silicon and othersemiconducting materials, the trimming and cutting of thick and thinfilms on semiconductors, the drilling of via holes in printed circuitboards, and the inspection of solder paste and component placement onprinted circuit boards and infrared beam scanning among many others.

FIG. 3 shows a conventional rotary bearing as might be used in areciprocating device such as a galvanometer. The bearing comprises aninner ring 2 and a concentric outer ring 4, the two rings beingseparated radially by balls 6 which roll in races defined by theopposing surfaces of the rings. In cross section, those races may havecylindrical or, more often, elliptical curvature so stresses are highestwhere the radially inner and outer poles of the balls contact thebottoms of the races. A lubricant is invariably provided between therings to minimize wear of the bearing parts. In a typical application,e.g., a shaft bearing for a galvanometer, the inner ring 2 may beangularly reciprocated relative to outer ring 4 about an axis A througha small angle θ of, say, 0°-15°. Since the balls 6 roll along the samesmall segments of the races, after only a few minutes of operation, thelubricant present in the reciprocating ball bearing is squeezed out ofthe high-pressure regions between the balls and the races, primarily atthe ball poles which contact the bottoms of the races. After only arelatively few reciprocations of the bearing ring 2 through an angle θof, say, 8°, small lubricant “hills” 8 build up in the races at theextreme ends of each ball's excursion along the races. Since the angle θthrough which the bearing rings 2 and 4 rotate relatively is always thesame, there is no mechanism to return the lubricant 8 to thehigh-pressure areas of the bearing between the ball poles and bottoms ofthe races where it is needed. Resultantly, those areas tend to wearexcessively.

If now the bearing rings 2 and 4 are caused to rotate relatively througha larger angle θ of, say 10° for one reason or another, the lubricanthills 8 deposited as aforesaid will increase the torque required torotate the bearing momentarily. Of course, once the new angularexcursion of the bearing ring 2 is established, the original lubricanthills 8 will disappear and reform on the races at the new extremes ofball travel. This process goes on continuously for the life of thelubricant in a bearing used in a random reciprocating application andmay not be noticed. Eventually however, all of the excess lubricant is“parked” at the extremes of the allowed bearing ball travel andthereafter has no great affect on day-to-day bearing operation. However,as noted above, the lubricant is not present in the high-pressure areasof the bearing where it is most needed. Consequently, excessive wearoccurs at those locations so that the bearing has a relatively shortuseful life. In fact, the aforesaid lubricant parking phenomenon is themain cause of bearing failure in most reciprocating bearings since,unless one bearing ring can rotate completely around relative to theother, the excess lubricant is effectively lost to the lubricantreplenishment process. This is why many failed bearings seem to haveplenty of lubricant remaining in them. The lubricant is in fact there,but it is not available at the high-pressure areas of the bearings whereit is most needed.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide along-lived ball bearing for reciprocating applications.

Another object of the invention is to provide an improved ball bearingfor the reciprocating shaft of a galvanometer.

Another object of the invention is to provide a bearing of this typewhich has improved wear characteristics.

A further object of the invention is to provide a method of prolongingthe life of a rotary ball bearing used in reciprocating applications.

Yet another object of the invention is to provide a method ofredistributing the lubricant in a rotary shaft bearing.

Other objects will, in part, be obvious and will, in part, appearhereinafter.

The invention accordingly comprises several steps and the relation ofone or more of said steps with respect to each of the others, and thearticle processing the features, properties and relation of elementswhich are exemplified in the following detailed description, and thescope of the invention will be indicated in the claims.

Briefly, my rotary bearing comprises at least three radially spacedconcentric rings separated by at least two circular arrays of ballslocated between adjacent rings. The opposing faces of the rings defineraces and the balls in each array are separated by a cage enabling theballs to roll along the races defined by those rings. Typically, ballsin the inner array are fewer and smaller than those in the outer arrayand the race crossections may have various shapes depending upon theparticular application. In any event, the intermediate ring acts as theouter ring with respect to the inner array of balls and as the innerring with respect to the outer array of balls.

Thus, the relationship between the rings and balls is no longer fixedbecause the intermediate ring can roll on both sets of ballsindependently of the inner and outer ring. As we shall see, this enableseach ball to travel around the circumferences of its races or trackseven though the inner and outer rings only reciprocate relatively. Toput it another way, if the inner and outer rings are fixed relatively,the intermediate ring is still free to rotate about the axis of thebearing thereby allowing the balls in each array to roll along theirrespective races independently of the angular relationship of the innerand outer rings.

It will be appreciated from the foregoing that as long as each array ofballs can roll along the entire circumference of the track defined bythe corresponding pair of adjacent rings, there will be no build-up oflubricant hills along the races defining that track. Because therecirculating arrays of balls will constantly redistribute or replenishthe lubricant along the races, particularly at the bottoms of the raceswhere it is most needed, bearing wear will be minimized.

An important aspect of the present invention, then, is to assure thatthe intermediate ring does rotate when the bearing is in use. This maybe done by mechanical means, e.g., by periodically engaging and rotatingthe intermediate ring using external means. However, it is mostpreferably accomplished by utilizing a ratcheting effect produced by thebearing lubricant on the intermediate ring when the inner and outerrings are reciprocated relatively beyond the normal angular excursion.As we shall see, this results in the bearing lubricant beingrecirculated around the circumferences of the bearing races so that thelubricant is always present at the bottoms of the races where stressesare maximum, thereby greatly increasing the life expectancy of thebearing.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be made to the following detailed description taken inconnection with the accompanying drawing, in which:

FIG. 1 is a perspective view of a galvanometer incorporating a bearingmade in accordance with the present invention;

FIG. 2 is a cross-sectional view of the bearing in FIG. 1;

FIG. 3, already described, is a diagrammatic view illustrating theoperation of a conventional bearing, and

FIG. 4 is a similar view showing the operation of the FIG. 2 bearing.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

Refer now to FIG. 1 which shows a galvanometer comprising a generallycylindrical housing 10 which rotatably supports a shaft 12. One end ofthe shaft projects from the housing and supports a small mirror 16. Theopposite end of the shaft is connected to the armature of an electricmotor 18 mounted within the housing. Shaft 12 is rotatably supportedwithin the housing by at least one rotary ball bearing 20 made inaccordance with this invention so that the shaft can be rotated relativeto the housing by motor 18. At least one other similar bearing (notshown) may located along shaft 12.

As in any conventional galvanometer, motor 18 is controlled by signalsfrom a control 22 so as to angularly reciprocate shaft 12 through anangle θ which may vary from, say, 0° to 15°. Thus, if the mirror 16 isilluminated by a light beam L, the nutating mirror 16 will cause thelight beam reflected from the mirror to sweep through the angle 20 asshown in that figure. A conventional rotary ball bearing used in agalvanometer such as this or in any other reciprocating applicationwould suffer the disadvantages described at the outset. To avoid this,each bearing 20 has the construction depicted in FIG. 2.

As shown is FIG. 2, bearing 20 comprises an outer ring 22 which may beflanged as shown, or not, for seating in housing 10 (FIG. 1), an innerring 24 for engaging around shaft 12 and an intermediate ring 26situated between the outer and inner rings, all of the rings beingconcentric to axis A. The intermediate ring 26 is illustrated as beingcomposed of two parts. However, it could just as well be a single part.The outer ring 22 and intermediate ring 26 are separated by a circulararray of balls 28. Balls 28 roll along a race 22 a on the radially innersurface of outer ring 22 and a race 26 a on the radially outer surfaceof intermediate ring 26. The two races 22 a and 26 a define a circulartrack for balls 28, the balls 28 being spaced apart along their track bya more or less conventional cage 32.

The inner ring 24 and intermediate ring 26 are spaced apart by a secondarray of balls 34 which roll along a race 24 a on the radially outersurface of inner ring 24 and a race 26 b in the radially inner surfaceof intermediate ring 26. Balls 34 are retained in place by the usualcage 36. A typical viscous lubricant such as oil or grease is present inthe spaces between the rings as indicated at 38 in FIG. 2.

When motor 18 reciprocates shaft 12 and mirror 16 through an angle θ,the inner ring 24 reciprocates through that same angle, while the outerring 22 remains stationary, being fixed to housing 10.

In a three-ring bearing such as bearing 20, the balls 34 in the innerarray are smaller than balls 28 in the outer array and there are fewerof them. Therefore, the bearings will normally display a lower operatingtorque between the inner ring 24 and the intermediate ring 26 as aresult of the smaller number of balls in the inner array of balls 34 andthe shorter radius of action of those balls. As a result, the shaft 12will tend to reciprocate the inner ring 24 through the angle θ, whilethe intermediate ring 26 remains stationary. Thus, each ball 34 willroll back and forth along its track between rings 24 and 26 a shortdistance as indicated by the arrow B in FIG. 4. Therefore, as discussedabove, hills of lubricant could build up in the races at the oppositeends of each ball's travel path as discussed in connection with FIG. 3.However, if motor 18 (FIG. 1) is controlled by controller 22 so as toperiodically rotate shaft 12 through an angular increment Δθ beyond theusual angle θ (i.e., θ+Δθ) as indicated in FIG. 4, the torque increasecaused by the balls 34 contacting the lubricant hills will cause theinner ring 24/intermediate ring 26 operating torque to exceed theintermediate ring 26/outer ring 22 operating torque momentarily, withthe result that the intermediate ring 26 will rotate with respect to thestationary outer ring 22 as shown by the arrow C in FIG. 4.

As soon as motor 18 resumes reciprocating shaft 12 at the original angleθ, the inner ring 24 will again rotate with respect to the intermediatering 26, that ring again being stationary with respect to the outerring. However, since the intermediate ring 26 did rotate momentarily asaforesaid, the balls 34 will now rotate along different sectors of theirraces 24 a and 26 b than they did originally.

The same is true with respect to the outer array of balls 28. In otherwords, due to the aforesaid rotation of intermediate ring 26 as shown atC, balls 28 will roll along slightly different travel paths in theirraces 22 a and 26 a than they did before shaft 12 was rotated throughthe angular increment Δθ. Any hills of parked lubricant 38 formedoriginally on recess 22 a, 26 a will be rolled flat by the balls 28 andthe lubricant 38 will be carried into new rolling areas along thoseraces as a result of the contacts between the balls and the lubricanthills.

It will be appreciated from the foregoing, then, that by repeating theprocess of interrupting the normal, the maximum excursion of the,reciprocating motion of shaft 12 with a slightly larger angularexcursion of the shaft, the intermediate ring 26 will eventually make acomplete revolution about axis A. Resultantly, balls 28 and 34 will berecirculated in their respective races thereby effecting aredistribution of the lubricant 38 along the races so long as thegalvanometer is in operation. Thus, reserve lubricant present in the lowpressure zones between the balls and the races will constantly replenishthe lubricant film which is squeezed out of the operating high pressurezones at the boundaries of the ball poles and the bottoms of the races.

When operated as described above, a bearing with at least three ringswhen used in galvanometers or other devices which rotate a shaft overangles smaller than that which allows each bearing ball to rollcompletely around its circular track will have a long life expectancy ascompared with a conventional two-ring bearing. In the case of a typicalgalvanometer employing a two-ring bearing and with a maximum rotation ofthe inner ring with respect to the outer ring of 30°, but typically lessthan 15°, the life expectancy increase obtained by substituting athree-ring bearing operated in accordance with this invention should bein the order of three times.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description are efficiently attained. Also,certain changes may be made in carrying out the above method and in theconstruction set forth without departing from the scope of theinvention. For example, as shown in phantom in FIG. 2, one end of theintermediate ring 26 may be formed as a ring gear and be turnedintermittently by a spur gear 40 rotated by a step motor 42. Also, thebearing may include more than three rings. Therefore, it is intendedthat all matter contained in the above description or shown in theaccompanying drawing shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the inventiondescribed herein.

What is claimed is:
 1. A method of operating a rotary bearing of thetype having concentric inner, intermediate and outer rings separated byinner and outer circular arrays of balls which roll along tracks definedby the rings and are lubricated by a viscous lubricant, the methodcomprising angularly reciprocating the inner and outer rings relativelyabout an axis through a range of angles up to selected maximum angle toproduce lubricant hills spaced along the tracks between said balls, andrecurrently changing the relative angular reciprocating motion of theinner and outer rings so that the balls push against said hills toeffect, solely through the lubricant, a unidirectional movement of theintermediate ring about said axis whereby the balls recirculate aboutsaid axis and redistribute the lubricant along said tracks.